The recent practice in regard to implementing the IF filtering and gain functions in television signal receiving apparatus is to use a lumped or "block" filter preceding a gain-block amplifier comprising plural stages arranged in direct-coupled cascade within the confines of a monolithic integrated circuit (IC). No interstage tuning is used. The amplified IF signal from the gain-block IC amplifier is further detected substantially within the confines of the monolithic integrated circuit, to emerge from the IC as baseband composite video signal and as sound IF at 4.5 MHz. The amplified IF signal is filtered from these signals to reduce the likelihood of regeneration causing the gain-block amplifier to oscillate.
The "block" filter is commonly a surface-acoustic-wave (SAW) filter used to provide the entire passband shape and adjacent channel attenuation required by a television receiver. Additional information on SAW filters and on block filtering and amplification may be found, for example, in Chapter 13 of the book TELEVISION ENGINEERING HANDBOOK; K. Blair Benson, Editor in Chief; McGraw-Hill Book Company, New York; 1986.
There are conflicting requirements on video IF amplification inasfar as best picture reception and best sound reception are concerned, which conflicting requirements are more difficult to resolve when a block filter preceding a gain-block amplifier is used to provide IF amplification. Good picture reception, with full resolution capability in the direction of horizontal sweep, requires that the IF signal as supplied to the video detector does not include response to in-channel sound carrier. Accordingly, in television signal receiving apparatus using discrete amplifying devices with interstage tuning, an in-channel sound trap commonly precedes the video detector. Good picture reception further requires that the IF signal as supplied to the video detector does not include response to adjacent-channel sound carrier, which response intrudes "sound beats" into the picture. In television signal receiving apparatus using discrete amplifying devices with interstage tuning, an adjacent-channel sound trap precedes the video detector and provided high (40 dB or so) rejection over the band in which the adjacent-channel sound signal as translated to IF appeared. To obtain such rejection and still maintain acceptably linear phase response for composite video, the video carrier as translated to an IF has to be located on the edge of the adjacent-channel sound trap rejection response, and is normally attenuated 6 dB or so. When a block filter preceding a gain-block amplifier is used to provide IF amplification, the traps for adjacent-channel and in-channel sound have to be provided within the block filter preceding the gain-block amplifier providing amplified IF for direct coupling to the video detector, which can be used for the generation of intercarrier sound.
Sound reception in a TV receiver using intercarrier sound exhibits better signal-to-noise ratio, however, when the sound and video carriers as translated to IF are not attenuated respective to mid-band prior to their being mixed together to generated 4.5 MHz sound IF signal. To achieve better sound reception while adhering to the design concept of block filtering, a further block filter and gain-block amplifier cascade is used for intercarrier sound in the invention, in addition to the block filter and gain-block amplifier cascade used to drive the video detector. The two gain-block amplifiers are preferably constructed in substantially the same way within the confines of the same monolithic IC, the inventor points out, such that their operating characteristics track each other. Preferably each of the gain block amplifiers has a balanced conversion stage cascaded thereafter within the IC, so the frequency of its output signal as supplied from the IC differs from the frequency of its input signal as supplied to the IC. This reduces the risk of the gain block amplifier regeneratively oscillating. To further reduce the risk of regenerative interaction between the two gain-block amplifiers, one gain-block amplifier can provide balanced output signals responsive to single-ended input signal, and the other gain-block amplifier can provide single-ended output signal responsive to single-ended input signals. The tracking of the gain-control characteristics of the two gain-block amplifiers is particularly important, since it allows an automatic gain control signal developed by detecting the tips of synchronizing pulses from the video detector cascaded after one of the two gain-block amplifiers to be applied in parallel to both the gain-block amplifiers for controlling their gains similarly.
In television signal receiving apparatus of the type having only a single conversion before video detection, the IF amplifier is typically required to handle input signals ranging from about 50 microvolts to about 100 millivolts RMS, which represents a dynamic range of about 66 dB. U.S. patent application Ser. No. 07/940,220 filed Sep. 8, 1992 by Jack Rudolph Harford and Heung Bae Lee, entitled VARIABLE GAIN AMPLIFIER, describes gain-block stages suitable for use in the invention herein described and claimed. As is the case in the present application, U.S. patent application Ser. No. 07/940,220 has been assigned to Samsung Electronics co., Ltd., pursuant to obligations of the inventor(s) to assign his (their) invention(s) at the time the inventions were made. In these gain-block stages a 66 dB range of gain control can be achieved using only two gain-controlled voltage amplifier stages, which facilitates the tracking of the gain-control characteristics of the two gain-block amplifiers.